Single venipuncture needles have been used to cyclically remove untreated blood from a patient and return treated blood to a patient, e.g., in conjunction with fluid flow transfer devices such as dialyzers. The control cycles consist of the arterial phase and the venous phase. In the arterial phase the blood is pumped from the patient through the needle and an arterial line while a venous line is clamped shut. In the venous phase the blood is returned to the patient through the venous line and the needle while the arterial line is clamped shut.
One or two pumps can be used, and the pumps can be used continuously or intermittently. The pumps and clamps used to control the flow in past systems have been switched between the arterial and venous phases of the cycle by responding to maximum and minimum pressures (e.g., Schael U.S. Pat. No. 4,231,366; Kopp U.S. Pat. No. Re. 29,346), by switching after given periods of time (e.g., Willock et al. U.S. Pat. No. 4,063,554) or by using both pressure and time (e.g., Kopp U.S. Pat. No. Re. 29,346; Kopp U.S. Pat. No. 3,830,234).
The arterial and venous lines are typically made of flexible tubing that expands and contracts slightly with changes in pressure. There might be as much as four feet of flexible tubing between the clamps and the venipuncture needle, and immediately following switching of the clamps from the venous phase to the arterial phase, the extra volume of blood in the expanded venous tube can be pushed into the arterial tube, causing recirculation. Similarly, a small amount of treated blood can be transferred from the venous line to the arterial line during switching from the arterial phase to the venous phase. Stroke volume is the amount of blood removed from a patient during a single cycle. Some operators of single needle apparatus have been interested in knowing the stroke volume being achieved during operation to make sure that it is large enough, and they have in instances estimated it by measuring the length of the cycle with a stop watch and multiplying the time times the pumping rate of the blood pump.